A Colorful Way to Watch Evolution in Nebraska’s Sand Dunes

GRAND EXPERIMENT Scientists constructed eight enclosures — four on light sand, four on darker soil — stocked them with equal numbers of dark and light mice, and are watching what happens over time.Credit
Rowan Barrett

VALENTINE, Neb. — The nearest ocean is a thousand miles away. Yet across central Nebraska, peeking out from rolling hills of prairie grass, is sand — beige and reminiscent of a Caribbean beach. Nebraska’s sandhills are the largest sand dunes in North America, spreading for more than 20,000 square miles, over more than a quarter of the state. The sand hails from the Rocky Mountains: tiny specks of eroded quartz that blew east and landed here, 8,000 years ago.

For a team of evolutionary biologists and geneticists from Harvard University who spent part of their summer in Valentine, on the sandhills’ northern edge, the draw is not the quartz itself, but a particular mouse whose coat color matches it. They’re using pigmentation to study a central question of biology: How do organisms adapt to their environments?

Color has preoccupied naturalists for centuries, and was a popular subject for early genetics research because it’s so visually obvious. Today, the Harvard group, run by Hopi Hoekstra, professor and curator of mammals for the university’s Museum of Comparative Zoology, is studying color to piece together evidence of how genetic diversity occurs, and how natural selection acts on that diversity.

They want to understand the specific genetic mechanisms that lead to changes in physical appearance, and then how those physical changes affect an organism’s fitness — how likely it is to survive and reproduce. “Fitness is the most important concept in biology,” said Dr. Hoekstra. “But no one ever measures it.”

The deer mouse, Peromyscus maniculatus, is the most populous mammal on the continent, a versatile little rodent that can thrive in virtually any environment. Before the sandhills formed, Nebraska’s soil was dark — and so were its deer mice. Even today, most of the region’s mice have medium- to dark-brown coats, except for the ones that scurry around on the sand. Their fur tends toward tan or orangey-blond.

The reason seems obvious enough: Mice that have dark fur and live on light sand are easy pickings for hungry hawks and owls. A genetic mutation led to lighter fur, and the sandhills mice lucky enough to be born mutants were more likely to survive and breed. With each generation, the population of light mice would grow, while that of the dark mice would shrink, until eventually, the bulk of the sandhills mice sported lovely beige fur. It’s a textbook tale that any high school student could explain.

The only problem is, it might not be true. To Dr. Hoekstra and Rowan Barrett, a postdoctoral researcher in her lab, the fact that it makes sense says nothing about its scientific credibility. “People love to tell stories about how species adapt and evolve,” said Dr. Barrett, “but no one ever sees it happen.”

Thanks to an accumulation of knowledge about both the ecology and the genetics of the Nebraska deer mice, the animals offer a rare opportunity to directly test the story, and to measure the effect of environmental conditions on evolution. Several years ago, Dr. Hoekstra and another postdoctoral researcher, Catherine Linnen — now at the University of Kentucky — discovered that a specific gene controls much of the fur coloring in P. maniculatus by telling pigment-producing cells how long to spend making certain shades. More recently, they identified four different parts of the gene where mutations affect coat color, and showed that natural selection is involved in the color changes.

“It’s kind of fun to find the mutations,” said Dr. Hoekstra, “because that’s the basic material of change. But at a bigger level, there are two general things we can learn. One is about how changes in gene expression evolve. And the other is, does evolution occur through big leaps or the accumulation of small, gradual changes?”

To answer this for the specific case of the deer mice, the scientists are examining the mutations that arise in a group of wild mice, how they affect physical appearance, and how that gives certain individuals a better shot at reproducing.

Photo

Credit
Rowan Barrett

To watch evolution in action, Dr. Barrett, who has a reputation for devising grand, slightly crazy experiments, dreamed up a project: construct eight enclosures, four on the light sand and four on the darker soil, stock them with an equal number of dark and light mice and watch what happens over time.

At a reservoir 20 miles south of Valentine, in the heart of sandhill country, Dr. Barrett and a rotating crew from Cambridge built four pens out of galvanized steel sheets. Each enclosure is a 150-foot square with three-foot-high metal walls dug two feet into the ground, anchored by steel rods hammered deep into the sand. It’s designed to keep mice in, and other rodents and ground-dwelling predators — like rattlesnakes — out. East of town, on a ridge of alfalfa fields, Dr. Barrett built four additional pens, identical to the reservoir enclosures except for the soil, which is dark.

An error has occurred. Please try again later.

You are already subscribed to this email.

Each evening for nearly a month, the crew caught mice, setting live traps — long metal boxes that snap shut with the pressure of little rodent feet — at light and dark locations. Fanning out in a corn field, on sandy ridges along the highway, or near the enclosures themselves, the team would proceed methodically: open the trap, set it down, throw in a handful of sunflower seeds. Each morning at 6:30, before the heat of the day would cook any mice caught in the metal boxes, they would return and collect the traps.

The garage of their rented house functioned as a makeshift lab. The scientists would sedate each mouse with a few whiffs of isoflurane, and then weigh it, measure it, punch a tiny hole in its ear for visual identification, and measure its coat color using a spectrophotometer, an instrument that tracks light waves. They photographed each mouse beside a color chart, and inserted a tiny microchip between its shoulder blades. Finally — usually after a bit more isoflurane — they cut a snip off the end of each mouse’s tail, to collect its DNA. These they stored in vials in the kitchen, in a refrigerator vegetable bin.

In the evening, before the next round of trapping, they released the mice into the enclosures. Each enclosure will eventually house 100 mice. Dr. Barrett will return roughly every six weeks, setting traps inside the pens to check the frequencies of genetic variants and monitor changes. This will help the scientists understand how genes are linked to physical appearance, and how both are linked to fitness. “We may be able to detect that a gene is being favored not because it controls coat color but because it’s doing something else to the organism that’s having some correlated effect,” said Dr. Barrett.

That, in fact, is just what he found during his Ph.D. research at the University of British Columbia. There, Dr. Barrett studied the evolution of the three-spine stickleback, a three-inch-long fish that lives both in oceans and in freshwater lakes. In the oceans, the fish are covered with armor. But in lakes, they’re virtually armor-free.

There was an “adaptive story” about the sticklebacks, as there was with the deer mice. The same armor that had protected the ocean sticklebacks from larger fish proved deadly in the lakes, making them easier for predatory insects to catch. So over time, they shed the armor.

But that story turned out to be flawed. In fact, the armor gene was linked to a gene for rapid growth, which gave the lake fish an advantage by allowing them to breed earlier and better survive cold winters. Growth rate, not armor, was determining survival — and driving evolution.

The mouse color story, of course, could turn out to be true. And that, said Dr. Hoekstra, would be just as satisfying, because it would still help show the pace of evolution.

“People typically have this view of evolution as being an extremely gradual process,” said Dr. Barrett. “But you have situations like antibiotic resistance, and systems that have been exploited by humans, like overfishing, where you get incredibly rapid changes. So doing these experiments can give you an idea, if you push wild animals, how quickly could they potentially evolve? It allows you to make predictions about how things are going to change in the future.”

Peter R. Grant, an evolutionary biologist and professor emeritus at Princeton, said: “The research is important because it is revealing details of the process of evolution. Most investigators are unable to establish clear links between genotype, phenotype”—physical appearance— “and environment in nature. Hopi and Rowan are doing just that.”

A version of this article appears in print on August 9, 2011, on Page D4 of the New York edition with the headline: A Colorful Way to Watch Evolution in Nebraska’s Sand Dunes. Order Reprints|Today's Paper|Subscribe